Abnormal or excessive angiogenesis is responsible for or associated with a wide range of diseases, including cancer (particularly solid tumors), blindness, arthritis and many others; therefore it is thought that anti-angiogenesis therapies are potentially effective for treating such diseases. Current anti-angiogenic therapies focus mainly on antagonizing the activity of angiogenic factors, such as vascular endothelial growth factor (VEGF). While such therapeutic agents (e.g. anti-VEGF antibodies) have been shown to be effective in certain situations, repeated administration is required to maintain the inhibition of angiogenesis. Moreover, because angiogenesis is a complex process regulated by a large number of pro-angiogenic and anti-angiogenic factors, agents that target only the activity of a single angiogenic factor may not be sufficient to prevent angiogenesis in many situations. Thus, because current approaches in which activity of a single angiogenic factor is inhibited are often ineffective, there is a need for more effective anti-angiogenic therapies. Therapies that are able to inhibit angiogenesis by modulating the activities of multiple angiogenic factors, and those able to confer long-term effect without repeated treatment, are more desirable.
Pigment epithelium derived factor (PEDF) is a 50 kDa (403 amino acid) glycoprotein. It was initially found to be secreted by retinal pigment epithelial (RPE) cells and is a potent natural anti-angiogenic factor of the eye. Reduced levels of PEDF have been reported in cases of age-related macular degeneration (AMD), and overexpression of PEDF cDNA inhibited neovascularization in a mouse model of AMD. See, for example, Dawson, D. W. et al.(1999) Pigment epithelium-derived factor: a potent inhibitor of angiogenesis. Science 285(5425):245-248; Stellmach, V. et al. (2001) Prevention of ischemia-induced retinopathy by the natural ocular antiangiogenic agent pigment epithelium-derived factor. Proc. Natl. Acad. Sci. USA 98(5):2593-2597 and Mori, K. et al. (2002) AAV-mediated gene transfer of pigment epithelium-derived factor inhibits choroidal neovascularization. Invest. Ophthalmol. Vis. Sci. 43(6):1994-2000.
The anti-angiogenic function of PEDF has also been implicated in various cancers. Reduced PEDF level has been found to correlate with the metastatic phenotype of certain cancers, such as prostate cancer; and overexpression of PEDF inhibited tumor growth in xenograft models. See, for example, Halin S. et al. (2004) Decreased pigment epithelium-derived factor is associated with metastatic phenotype in human and rat prostate tumors. Cancer Res. 64(16):5664-71 and Abe R et al. (2004) Overexpression of pigment epithelium-derived factor decreases angiogenesis and inhibits the growth of human malignant melanoma cells in vivo. Am. J. Pathol. 164(4):1225-1232.
Because PEDF functions by inducing apoptosis of replicating endothelial cells, it is able to antagonize the activities of a number of different angiogenic factors that promote the proliferation of vascular endothelial cells, such as vascular endothelial growth factors (VEGFs), fibroblast growth factors (FGFs), and insulin-like growth factors (IGFs). Tombran-Tink, J. et al. (2003) Therapeutic prospects for PEDF: more than a promising angiogenesis inhibitor. Trends Mol. Med. 9(6):244-250. PEDF-based anti-angiogenesis therapy (i.e. activation of PEDF expression) is therefore likely to be more widely applicable, and more effective, than therapies in which expression of a single pro-angiogenic factor is inhibited (such as, e.g. VEGF antibody therapy).